1. Field of the Invention
This invention relates to zirconium and zirconium alloys, and in particular relates to processes for making purified zirconium for use in liner for reactor cladding.
2. Description of the Related Art
In the commercial production of zirconium and hafnium metal, the ore is generally initially subject to a chlorination step which produces a relatively impure, hafnium containing, zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated). The zirconium and hafnium containing material is then subjected to a number of purifying operations and also a complex hafnium separation operation. These operations result in purified oxides of zirconium and hafnium which, of course, are maintained separately. The purified oxides are then separately chlorinated. Zirconium and hafnium are commonly reduced from the chloride by means of a reducing metal (generally magnesium). Excess reducing metal and by-product salt, (e.g. magnesium and magnesium chloride) are removed from the so-called zirconium "sponge" by a distillation step. The zirconium metal is then generally double or triple vacuum arc melted to produce an ingot, which is then further processed (e.g. into zircaloy tubing for reactor fuel element cladding).
Ultrapure zirconium has been proposed for a liner for the inside surface of Zircaloy tubing for use as a cladding for reactor fuel, as described in, for example, U.S. Pat. No. 4,372,817 to Armijo et al. on Feb. 8, 1983. A similar use, but with moderate purity material, is proposed in U.S. Pat. No. 4,200,492 to Armijo et al. on Apr. 29, 1980.
Ultrapure zirconium has been produced in iodide cells by the so-called "crystal bar" process, (a very expensive process) as discussed, for example, in U.S. Pat. No. 4,368,072 issued to Siddall on Jan. 11, 1983. Material for lining cladding for reactor fuel elements by electron beam (EB) melting is disclosed in Japanese Patent Application No. 1979-144,789 by Kawakita et al., published June 8, 1981. That application discloses utilizing electron beam melting as the final melting, in a quite small laboratory (rather than commercial EB furnace.
Commercial reactors generally use either Zircaloy-2 or Zircaloy-4. The history of the development of Zircaloy-2 and Zircaloy-4 is summarized in: Kass, "The Development of the Zircaloys", ASTM Special Technical Publication No. 368 (1964), pages 3-27. Also of interest with respect to Zircaloy development are U.S. Pat. Nos. 2,772,964; 3,097,094; and 3,148,055. Zircaloy-2 is a zirconium alloy having about 1.2-1.7 weight percent (all percentages herein are in weight percent) tin, 0.07-0.20 percent iron, about 0.05-0.15 percent chromium, and about 0.03-0.08 percent nickel. Zircaloy-4 generally contains about 1.2-1.7 percent tin, about 0.18-0.24 percent iron, and about 0.07-0.13 percent chromium.
U.S. Pat. No. 4,675,153 is a cladding with a zirconium alloy liner having generally less alloying agent content than its conventional zirconium alloy outer layer (typically 0.2-0.6 Sn, 0.03-0.11 Fe, less than 0.02 Cr, less than 350 ppm oxygen, with the balance essentially Zr) and U.S. Pat. No. 4,613,479 is an example of a niobium zirconium alloy with less than 59% Zr (these two U.S. patents are hereby incorporated by reference).
EB (electron beam) melting of materials, including zirconium, has been discussed in a number of patents. EB melting has been used to consolidate crushed particles or chips in so-called hearth furnaces and to separate impurities by either overflowing floating inclusions (U.S. Pat. No. 4,190,404 to Drs et al. on Feb. 26, 1980) or to produce an electrode for arc melting (U.S. Pat. No. 4,108,644 to Walberg et al. on Aug. 22, 1978). A number of U.S. patents have used EB melting of powders or granules, often producing an ingot in a chilled mold. These powder melting EB patents include U.S. Pat. Nos. 2,942,098 to Smith on June 21, 1960; 2,960,331 to Hanks on Nov. 15, 1960; 0 to Hanks et al. on Dec. 6, 1960; 2,997,760 to Hanks et al. on Aug. 29, 1961; 2,935,395 to Smith on May 3, 1960; and 4,482,376 to Tarasescu et al. on Nov. 13, 1984. Electron beam zone refining using multiple passes is described in U.S. Pat. No. 3,615,345 to King on Oct. 26, 1971.
EB melting using a consumable feed "electrode" to produce an ingot collected in a chilled mold has also been discussed in a number of patents, including U.S. Pat. Nos. 3,087,211 to Howe on Apr. 30, 1963; 3,226,223 to Bussard et al. on Dec. 28, 1965; 2,880,483 to Hanks et al. on Apr. 7, 1959; and 4,130,416 to Zaboronok et al. on Dec. 19, 1978. U.S. Pat. No. 3,219,435 to Gruber et al. on Nov. 23, 1965 shows a commercial type EB furnace utilizing multiple beams. Typically the beams are directed to the surface of the molten pool and are continually swept across the pool surface to avoid overheating of any single portion of the pool surface. U.S. Pat. No. 3,091,525 to D'A. Hunt on May 28, 1963 describes adding a small amount of zirconium, for example, to hafnium, for example and melting in an EB furnace to deoxidize the hafnium.